Electric linear-actuator pumping system

ABSTRACT

A pumping system includes at least three electrically driven pumping units. Each pumping unit includes a linear actuator driven by at least one electric motor and driving a shaft. Pistons on each end of the shaft move within fixed cylinders to draw fluid from an inlet and expel the fluid to an outlet. Each pumping unit draws fluid at one end while expelling an equal quantity of fluid at the opposite end. The shaft velocity is controlled such that a total flow of the pumping system is constant, eliminating flow ripple.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Applications 62/849,405 filed May 17, 2019 and 62/934,268 filed Nov. 12, 2019, the entire disclosures of which are incorporated by reference herein.

TECHNICAL FIELD

The disclosure is applicable to the field of pumping systems. More particularly, the disclosure applies to a pumping system utilizing an electric linear actuator.

BACKGROUND

The practice of fracking has greatly increased the amount of oil and natural gas produced within the United States. Fracking involves pumping large quantities of fluid into wells. Conventionally, this is accomplished by reciprocating pumps driven by diesel engines. Due to the availability of natural gas on site, it would be preferable to use electric power from natural gas turbine driven generators.

Conventional fracking pumps utilize a crankshaft and connecting rod mechanism to convert rotational motion into axial reciprocating motion of a piston. Each cycle of the piston produces a pulse of flow, with the flow rate during each pulse being a function of the crankshaft and connecting rod geometry. Use of a large number of pistons with offset pulses allows the total flow rate to be partially smoothed out, but never completely constant. The variations in flow rate are called flow ripple. Flow ripple causes pressure pulses that increase failure rates of various components in the system. Also, for a given system size, such a pump has a very limited stroke distance. Therefore, many strokes per unit time are required to achieve a desired flow rate. This increases wear on valves which must open and close once per stroke.

SUMMARY

A pumping system includes a fluid inlet, a fluid outlet, and three or more pumping units. Each pumping unit includes a threaded shaft, an axially fixed nut, and one or more electric motors. The threaded shaft is fixed to a piston such that axial movement of the piston in a first direction draws fluid from the inlet and axial movement of the piston in a second direction expels fluid to the outlet. The piston may be fixed to one end of the shaft. A second piston may be affixed to an opposite end of the shaft such that axial movement of the second piston in the first direction expels fluid to the outlet and axial movement of the second piston in the second direction draws fluid from the inlet. The nut engages the threaded shaft such that rotation of the nut results in proportional axial movement of the shaft. The electric motors are drivably connected to the nut. For example, the one or more motors may include a ring gear fixed to the nut, a plurality of rotors, and a plurality of stators. Each of the rotors may be fixed to a pinion gear engaging the ring gear. Each may be configured to establish a magnetic field to exert torque on a respective rotor in response to an electric current. A controller may be programmed to command the pumping units to produce a repeating trapezoidal flow rate comprising increasing stages, steady stages, and decreasing stages. Pumping units in an increasing stage may offset pumping units in a decreasing stage such that a total flow rate is constant. The controller may include a plurality of local controllers each controlling one of the three or more pumping units. One of the local controllers may be designated as a master controller to coordinates the remaining local controllers.

A method of controlling a group of at least three linear actuator pumping units includes commanding a repeating pattern of shaft speeds of each of the pumping units and offsetting the pumping unit shaft speeds such that a total pump flow is constant. The repeating pattern of shafts speeds includes: travel in a first direction at a first speed for a first interval, gradually change speed until the shaft is moving in a second direction at a second speed equal in magnitude to the first speed, travel at the second speed for second interval equal in duration to the first interval, and then gradually change the speed until the shaft is moving at the first fixed speed. A number of units currently changing shaft speed from the first speed to the second shaft speed may be maintained equal to a number of units currently changing shaft speed from the second speed to the first speed. More particularly, the speeds of the shafts may be offset such that at any point in time, exactly one shaft is changing from the first speed to the second speed and exactly one shaft is changing from the second speed to the first speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a pumping system with three pumping units.

FIG. 2 is a schematic diagram of a linear actuator-based pumping unit suitable for use in the pumping system of FIG. 1.

FIG. 3 is a cut-away pictorial view of the pumping unit of FIG. 2.

FIG. 4 is a graphical representation of the speed and flow rate of a pumping unit when operated such that the total flow for the pumping system is constant.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It should be appreciated that like drawing numbers appearing in different drawing views identify identical, or functionally similar, structural elements. Also, it is to be understood that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the embodiments. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.

The terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the following example methods, devices, and materials are now described.

FIG. 1 schematically illustrates an electric linear-actuator pumping system. The exemplary pumping system uses three pumping units 10A, 10B, and 10C. The number of pumping units may vary. The structure of each pumping unit is described in detail below. Each pumping unit uses electrical power to draw a fluid from a source of unpressurized fluid 12 and deliver the fluid at increased pressure to a fluid outlet 14.

FIG. 2 schematically illustrates the internal structure of each of the pumping units 10A, 10B, and 10C. Each pumping unit includes an electric linear actuator 20 which utilizes electrical power to translate a shaft 24. The shaft 24 may be hollow to reduce weight. Pumping chambers 26 and 28 are located at opposite ends of shaft 24. Each pumping chamber draws fluid from the source of unpressured fluid 12 when the shaft is moving in one direction and delivers pressurized fluid to the output 14 when the shaft is moving the opposite direction. The two pumping chambers 26 and 28 are arranged such that one is drawing in unpressurized fluid while the other is expelling pressurized fluid.

The electric linear actuator 20 includes a plurality of electric motors each having a fixed stator 30 and a rotatable rotor 32. Electric power is provided to windings of the stators 30, creating magnetic forces that exert torque on the rotors 32. The motors may be, for example, an alternating current motor such as a permanent magnet synchronous motor. With a synchronous alternating current motor, the rotational speed of the rotor is adjusted by adjusting the frequency of the electric current using an inverter. With other types of motors, a speed or position feedback signal may be required.

Each rotor 32 is fixedly coupled to pinion gear 36. Each pinion gear engages a ring gear 38. FIG. 2 illustrates a ring gear with external gear teeth, but a ring gear with internal gear teeth is also possible. Ring gear 38 is fixedly coupled to a nut 34 of a planetary screw drive mechanism as described, for example, in U.S. Pat. No. 9,267,588 which is incorporated by reference herein. The nut 34 of the screw drive mechanism engages external threads of shaft 24. Rotation of nut 34 in response to rotation of the rotors 32 causes shaft 24 to displace along its axis.

A piston 40 is attached to each end of shaft 24. The piston slides within a cylinder 42. A volume 44 is defined between the piston 40 and a closed end of the cylinder 42. This volume increases as the piston moves away from the closed end of the cylinder and decreases as the piston moves toward the closed end of the cylinder. When the volume is increasing, valve 46 is open to allow unpressurized fluid to flow into the volume and valve 48 is closed to isolate the pressurized outlet from the volume. Movement of the piston creates a vacuum in the cylinder and atmospheric pressure forces the unpressurized fluid into that space. In some embodiments, the inlet fluid may be slightly pressurized. When the volume is decreasing, the axial force exerted on the shaft 24 is transmitted to the fluid in the volume to pressurize the volume. Valve 48 is open to allow the pressurized fluid to flow to the outlet. Valve 46 is closed to prevent the pressurized fluid from flowing back toward the inlet. Valves 46 and 48 may be, for example, passive check valves.

FIG. 3 is a cut-away pictorial view of a pumping unit.

Each pumping unit may have a local control unit. One of the local control units may be designated as a master control unit. The master control unit continually monitors a control signal or multiple control signals from a sitewide controller which controls multiple pumping systems. These signals indicate a desired flow rate and pressure from the pumping system. The master controller calculates a trapezoidal motion profile for each actuator unit in the local pump system, the sum of which meets the demand, and commands each local controller accordingly. The local controllers utilize various types of feedback signals which may include: back-emf voltage from the motors, current supplied to the motors, linear position sensors attached to the reciprocating portion of the pumps, rotary position sensors on the integrated nuts, pressure sensors in the fluid chambers of the pumps, strain sensors on the load-bearing elements of the pumps, and condition monitoring sensors in the bearings. The local controllers adjust the motion of its actuator's motors to achieve close adherence to the commanded motion profile, even sharing of torque load on each motor within an actuator unit, and protection from damaging conditions such as cavitation, low pressure, and incomplete fillage. The master controller adjusts the motion profiles of each actuator unit in the local group to achieve even wear and maximum life of each unit, real-time compensation for flow ripple (as discussed below), and special operating conditions as instructed by sitewide controller such as: pulsation or shockwave generation, ramp up/down, and/or idle. The local controller relays real-time operating parameters (position, velocity, status) to master controller. The master controller relays real-time operating parameters (position, velocity, status) to sitewide controller.

The top portion of FIG. 4 illustrates the velocity of shaft 24 as a function of time. During a first phase 60, the shaft moves in a positive direction at a steady speed. During a second phase 62, the shaft slows down at a steady rate. During the middle of the second phase, the shaft changes direction. During a third phase 64, the shaft moves in a negative direction at a steady speed, which is equal in magnitude to the speed of the first phase. Finally, during a fourth phase 66, the shaft accelerates at a steady rate equal to the rate of deceleration of the second phase. At the end of the fourth phase, the shaft has returned to its original position and speed and the process is repeated.

The bottom portion of FIG. 4 illustrates the fluid flow rate as a function of time. Note that the flow rate is proportional to the absolute value of the velocity. When the shaft is moving in a forward direction, flow is provided to the outlet from one of the pumping chambers. When the shaft is moving in a negative direction, flow is provided by the other pumping chamber. During the first phase 60, a constant flow rate 70 is provided by pumping chamber 26. During the first half of the second phase 62, the flow rate from pumping chamber 26 decreases to zero as shown at 72. During the second half of phase 62, the flow rate from pumping chamber 24 increases as shown at 74. During the third phase 64, a constant flow rate 76 is provided by pumping chamber 24. During the first half of the fourth phase 66, the flow rate from pumping chamber 24 decreases to zero as shown at 78. During the second half of phase 66, the flow rate from pumping chamber 26 increases as shown at 80.

With three pumping units, these phases are staggered to maintain constant total flow. At any given time, one pumping unit is operating in either phase 70 or 76, another pumping unit is operating in either phase 72 or 78, and a third pumping unit is operating in either phase 74 or 80. With three total pumping units, the length of phase 60 and 64 should be half as long as the length of phases 62 and 66. With different numbers of pumping units, the relative durations of the phases may be adjusted such that one unit is always in a declining flow phase and one unit is always in an increasing flow phase.

In addition to establishing a constant flow rate, the pumping system described above offers several advantages. Each of the pumping units has a relatively long stroke relative to its overall size. As a result, the valves do not need to open and close as often as they would for a shorter stroke pump at the same average flow rate. This improves the durability of the valves. Furthermore, the pumping system can continue to operate with one of the pumping units offline which simplifies maintenance.

The local controllers may use information gathered from the sensors to calculate remaining life of its actuator unit. In the event of a failure or emergency, local controller may be programmed to stop motion of its actuator unit. In the event of a failure of one of the actuator units, the master controller may attempt to compensate for flow ripple and loss of pressure and/or flow rate by adjusting the motion profiles of the remaining pumping units.

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the disclosure that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. As such, to the extent any embodiments are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics, these embodiments are not outside the scope of the disclosure and can be desirable for particular applications. 

What is claimed is:
 1. A pumping system comprising: a fluid inlet; a fluid outlet; and three or more pumping units, each pumping unit having a threaded shaft fixed to a piston such that axial movement of the piston in a first direction draws fluid from the inlet and axial movement of the piston in a second direction expels fluid to the outlet, an axially fixed nut engaging the threaded shaft such that rotation of the nut results in proportional axial movement of the shaft, and one or more electric motors drivably connected to the nut.
 2. The pumping system of claim 1 wherein: the piston is fixed to one end of the shaft; and a second piston is affixed to an opposite end of the shaft such that axial movement of the second piston in the first direction expels fluid to the outlet and axial movement of the second piston in the second direction draws fluid from the inlet.
 3. The pumping system of claim 1 wherein each of the one or more electric motors drivably connected to the nut comprises: a ring gear fixed to the nut; a plurality of rotors, each fixed to a pinion gear engaging the ring gear, and a plurality of stators, each configured to establish a magnetic field to exert torque on a respective rotor in response to an electric current.
 4. The pumping system of claim 1 further comprising a controller programmed to command the pumping units to produce a repeating trapezoidal flow rate comprising increasing stages, steady stages, and decreasing stages, wherein pumping units in an increasing stage offset pumping units in a decreasing stage such that a total flow rate is constant.
 5. The pumping system of claim 4 wherein the controller comprises a plurality of local controllers each controlling one of the three or more pumping units.
 6. The pumping system of claim 5 wherein one of the plurality of local controllers is designated as a master controller and coordinates the remaining local controllers.
 7. A pumping system comprising: a fluid inlet; a fluid outlet; and three or more pumping units, each pumping unit having a threaded shaft fixed to a piston such that axial movement of the piston in a first direction draws fluid from the inlet and axial movement of the piston in a second direction expels fluid to the outlet, an axially fixed nut having internal threads, a plurality of threaded rollers engaging the threads of the nut and the threaded shaft such that rotation of the nut results in proportional axial movement of the shaft, a ring gear fixed to the nut, a plurality of rotors, each fixed to a pinion gear engaging the ring gear, and a plurality of stators, each configured to establish a magnetic field to exert torque on a respective rotor in response to an electric current.
 8. The pumping system of claim 7 wherein: the piston is fixed to one end of the shaft; and a second piston is affixed to an opposite end of the shaft such that axial movement of the second piston in the first direction expels fluid to the outlet and axial movement of the second piston in the second direction draws fluid from the inlet.
 9. The pumping system of claim 7 further comprising a controller programmed to command the pumping units to produce a repeating trapezoidal flow rate comprising increasing stages, steady stages, and decreasing stages, wherein pumping units in an increasing stage offset pumping units in a decreasing stage such that a total flow rate is constant.
 10. The pumping system of claim 9 wherein the controller comprises a plurality of local controllers each controlling one of the three or more pumping units.
 11. The pumping system of claim 10 wherein one of the plurality of local controllers is designated as a master controller and coordinates the remaining local controllers.
 12. A method of controlling a group of at least three linear actuator pumping units, the method comprising: commanding a shaft of each of the at least three pumping units to repeatedly: travel in a first direction at a first speed for a first interval, gradually change a shaft speed until the shaft is moving in a second direction at a second speed equal in magnitude to the first speed, travel at the second speed for second interval equal in duration to the first interval, and then gradually change the shaft speed until the shaft is moving at the first fixed speed; and offsetting the speeds of the shafts of the at least three pumping units such that a total pump flow rate of the at least three pumping units is constant.
 13. The method of claim 12 wherein a number of units currently changing shaft speed from the first speed to the second shaft speed is maintained equal to a number of units currently changing shaft speed from the second speed to the first speed.
 14. The method of claim 12 wherein the speeds of the shafts are offset such that at any point in time, exactly one shaft is changing from the first speed to the second speed and exactly one shaft is changing from the second speed to the first speed. 